The present invention relates to data signal time measurements, and more particularly to a transition-density based time measurement method.
In a high-speed data transfer system it is not uncommon for a data signal to have a large amount of inter-symbol interference (ISI) and other forms of jitter at a receiver. When the jitter on the data signal occupies a significant percentage of the data period, accurate time measurements are a challenging task. This happens infrequently in low-speed systems, but is not unusual in high-speed systems.
When the data signal has a large amount of jitter, which could be ISI, periodic jitter (PJ), duty-cycle-distortion (DCD), random jitter (RJ) or a combination of these jitter components, the data time measurement method based on pattern match, as described in U.S. Pat. No. 6,356,850 entitled “Method and Apparatus for Jitter Analysis” by WaveCrest Corporation, has difficulty making a correct pattern match because of the high noise level. Getting a good estimation of the data period for the pattern match algorithm to start with is also difficult in this case. Another data time measurement method is based on a pulse width histogram. This method estimates the data period from samples around the first significant pulse in a histogram of pulse widths under the assumption that these samples represent a width of one unit interval. The histogram-based method has difficulty avoiding bias when the data signal has large jitter, particularly large ISI.
Both methods are based on variables that may be significantly affected by jitter. Pattern match methods need to know the specific bit sequence of the pattern. For a 23-bit pseudo-random binary sequence (PRBS) with a pattern length of roughly eight million, pattern match is quite expensive computationally.
Transition-Density (TD) is the probability of a transition occurring. Normally TD may be seen as the average transition density. Data streams used in real data communications systems typically have a transition density of 50%—see “Fibre Channel —Methodologies for Jitter and Signal Quality Specification—MJSQ”, Technical Committee T11 of the International Committee for Information Technology Standards, Project 1316-DT.
What is desired is a data time measurement method that provides accurate time measurement on non-return-to-zero (NRZ) data signals that have large jitter.